Desiccation method and apparatus



Au8 27, 1946. N. v. HAYES ETAL l DEsIccATIoN METHOD AND APPARATUSFileduarch 9, 1944 INVENTORS www V- HAYES.

:LLI/u1 B. /luqgg (k2/6M ATTORNEY 'w se of the moisture of thesubstance.

Patented Aug. 27., 1945 DESICCATION METHOD AND APPARATUS setteApplication March 9, 1944, Serial No. 525,796

.t Claims.

E This invention relates to methods and apparatus for desiccatingsubstances such as foods, chemicals, plasma and the like, and moreparticularly to such methods and apparatus in whichthe desiccation isperformed at low temperature and under low pressure. y

Desiccation at atmospheric pressures requires subjecting the substancetoelevated temperatures for long periods of time in order to evaporate themoisture. In the case of substances such as those previously mentioned,the consequence of such exposure to elevated temperatures is a seriousdeterioration of the product. 'Vitamins in foods are destroyed and tasteis impaired. Vital components of many chemicals and plasmas are impairedor destroyed by such treatment.

' g By reducing the free air pressure on the substance, the temperatureat which Athe moisture content of the substance will evaporate is alsoreduced. Thus if the pressure is reducedA to a sufficiently low value,the moisture will evaporate at a temperature low enough so thatdesiccation may be effected without impairment of the qualities of thesubstance. For most substances of the character above mentioned, thattemperature is C. or lower, that is, at or below the freezing point Toproduce eective desiccation byevaporation or sublimation at such lowtemperatures usually calls for the maintenance on the'substance oi atotal pressure below-3 mm. Hg.

When a substance undergoes ,desiccation in a closed system, the vaporpressure of the evaporating moisture tends to increase the pressure inthe system. In order to maintain a low total pressure duringdesiccation, therefore, it is necessary to remove or pump the vaporssubstantially as fast as they form. Mechanical pumps, which can beutilized to provide the initial low Afree air pressure in the system, donot have the pumping speed necessary to remove evaporating vapors withsufficient rapidity to maintain the desired low pressure afterdesiccation starts. v'

In application of Richard S. Morse, Serial No. 483,274, filed April 16,1943, there are disclosed a method and apparatus for vacuum desiccationin which the evaporating vapors are rapidly and eilectively pumped byfreezing them on a refrigerated condenser withinthe system, theedlciency of the condenser being maintained through continual removal ofthe ice of condensed vapors from its surface. In this way it is possibleto maintain during desiccation of a large volume of a-substance a lowpressure on the substance, considerably below 3 mm. Hg., producedinitially in 2 the system by ordinary mechanical pumping means.

. It is the object of this invention to provide improvements upon themethod and apparatus of the aforesaid application by means of whichsubstantially all of the vapors of evaporation may be effectivelyeliminated from the system in advance of the mechanical pump so that theamount of vapor pumping required of the mechanical pump isreduced tozero or anegligible value and the eihciency of the system isconsiderably improved.

We have -found that if ice removed from the condenser in the aforesaidapparatus is permitted to resublime in the system, the eiliciency of theapparatus is reduced and that this undesirable condition can besubstantially completely eliminated by a controlled refrigeration ofthat part of the system, between the condenser and themechanical pump,which receives the ice removed from the surface of the condenser.

This refrigeration may be effected by circulating a refrigeratingsubstance overI the Walls of that part of the pumping passage in whichice' from the condenser is received to maintain a controlled lowtemperature in those walls. Curiously, however, if that temperature isappreciably lower than the temperature of the condenser, the vaporsemanating from the substance undergoing desiccation will pass throughthe condenser without condensing and then freeze preferentially 4,uponthe colder surface of the walls of the passage succeeding the condenser.Under the conditions in'- volved, the vapors evidently seek out the'coldest condensing surface exposed to their path, regardless of relativedistance from their source. Such preferential freezing of the vaporsupon the pumping passage walls beyond the condenser is,

of course, undesirable.

We have found that this last mentioned dilculty can be overcome bycarefully controlling the refrigeration of those vvalls4 to maintaintheir temperature at or above the temperature of the condenser. However,to effectively accomplish our purpose of preventing resublimation of theice in this part of the passage, we have found that the temperature ofthe passage walls should not greatly exceed that of the condenser andpreferably is not more than 10 C. above the temperature of thecondenser.

As an alternative or supplementary procedure to that just described,preferably supplemental, we may prevent vapors of resubliming ice fromreaching the mechanical pump by refreezing them. l

To this end we provide a. second or auxiliary consuch refrigerant.

denser between the first condenser and the mechanical pump, this secondcondenser being kept at a temperature equal to or slightly above that ofthe first condenser to avoid preferential, initial condensation thereonof vapors from the desiccating substance. As in the case of the firstcondenser, the ice of condensed vapors is scraped or otherwise removedfrom the surface of ,this second condenser, and this ice is returned tothe part of the pumping passage between the two condensers where the iceremoved from the first condenser is collected for. withdrawal.

The invention will be further describedwith reference to theaccompanying drawing wherein Fig. 1 is a view partly in section, partlyin side elevation, ofa preferred form of apparatus of the invention, andFig. 2* is a transverse section view on the line 2-2 of Fig. l.

Referring to Fig. 1 of the drawing, an hermetically sealable vessel Ireceives the substance to be desiccated which may be fed thereto througha valved inlet duct I2. 'I'he vessel I0 is shown as provided in itsinterior with an agitator device I4 operated by an external motor I6 toagitate and increase the surface exposure of the substance, theoperating shaft extending through a gland I I which f orms a pressuretight seal around the shaft. The substance in the vessel may be throughthe upper end of duct section 24 where it is operatively connected witha motor 46, the upper end of section 24 being hermetically sealed aboutthe shaft 38 by means of a cover 48 pro'- vided with a gland 50 throughwhich the shaft passes. A pair of blade receiving arms 52 extendoppositely from rotor 36 to adjacent the condenser surface, eachprovided with a longitudinal slot in which the inner end of a ilatscraper blade 54 is received and fastened by set screws 56. The outeredge oi' each blade 54 has a beveled scraping edge 58 which extends asclose as is practicable to the condenser surface, about V64, in chclearance being sumcient. The blades and their support extend the fulllength ofthe condenser surface of section 26.

The lower end of condenser section 26. is connected to a port 60 in thetop of an hermetically heated in any suitable manner to preventexcessive lowering of its temperature by rapid'evaporation of moisturetherefrom. Vessel I0 is provided with a port I8 which is connected by intermediate connections hereinafter described with the pumping port 2|)of a mechanical vacuum pump (not shown) which may be of usualconstruction. Ports .I8 and 26 and intermediate connections form apumping passage through which air may be exhausted from the vessel Iiiby the mechanical pump. l

Port I8 is connected by a valved curved duct section 22 with a branch 23of a conduit section 24 which communicates with the upper end of avertically disposed conduit section 26. Section `26 forms the main vaporcondenser of the apparatus and to that end is provided with a Jacket 28which is supplied with a suitable refrigerant,

such as carbon dioxide gas, through pipes 60, 82r

connected thereto and to a suitable source of 26 is also provided withprojecting metal fins 34 over which the refrigerant circulates withinjacket 28 and which aid effective heat transmission from. thecylindrical inner surface ofthe section 26, which constitutes thecondensing surface, to the refrigerant.'

- Mechanism is provided for continually remov- The outer surface ofsection ing ice from the condenser surface of section 26.

As in the apparatus disclosed in application Serial No. 483,274previously mentioned, this mechanism is in the form of a continuouslyoperating scraper but its form diiers from, and it is an improvementupon, that of the aforesaid application. As shown in Figs. 1 and 2. thismechanism comprises a rotor 36 extending longitudi- Lally centrallythrough the section 26 and rotat-a ably mounted therein by a pair ofshafts 38 and 40 fastened, respectively, in suitable sockets in theupperand lower ends of the rotor, these shafts in turn being rotatablymounted in thrust bearings in the hubs ofv a pair of spiders 42, 44which are respectively fastened between` the upper end of section 26 andthe preceding portion 24 of the pumping passage, and between the lowerend of passage. The upper end of shaft 3-8 extends scalable icereceptable 62. The sides and base of the receptacle are provided with ajacket 64 within which there may be circulated a refrigerating fluid bymeans `of pipes 66, 61 connected to a suitable source of therefrigerant. Near the base of the receptacle is a port 68 extendingthrough the Areceptacle and jacket and outwardly thereof, through whichice may be periodically removed from the interior of the receptacle. Theinner end of port 68 is closed by a removable door 'I0 and its outer endis closed by a cover I2 removably fastened to the port by screws 14.Cover 12 is provided on its inner surface with a spring pressed piston'I6 which bears against a lug 'I8 on the outer face of door i6, pressingthe door into position with respect to an inner4 rim of the port. Agasket 86 provides a pressure tight seal between cover I2 and port 68.'Door "I6 may be effectively insulated from external heat by evacuatingthe space within port 68 between the door and cover l2 through a pipe 82which may be connected to the mechanical pump.

Receptacle 62 is provided in its base with a drain plug 84 and at itstop with a port 86 sealed by a cover 68 containing a sight glass throughwhich the condition of ice within the receptacle may be viewed.

A third port 66 in the top of the receptacle is connected with thepumping port 26 of the mechanical pump by means of a vertical conduitsection 92 fastened to port 9|)` and a T-shaped conduit section 66fastened to the top of section 92 and with its lateral branch connectedto port 28. Ports 96 and 26 and. sections 92 and 94 which form theoutlet pumping passage from receptacle 82 are all of substantially thesame,4

-tion 92 is mounted a scraper blade unit |64 operated by a motor |06.Inasmuch as this assembly, its mounting and operating connections are,as shown, identical except for smaller size to those of the main scraperunit previously described, a detailed description thereof isunnecessary.

It will be understood that the various sections making up the vpumpingpassage are sealed together in pressure tight relation and that pressuretight seals such as glands are provided around operating parts extendinginto the passage.

In operation, the kettle I0 is charged with a quantity of substance tobe desiccated and the system from the inlet tothe kettle to the inlettothe mechanical pump is hermetically sealed. The mechanical pump isstarted, reducing the free air pressure in the system to a point belowthe vapor pressure of the moisture of the substance to be desiccated.This causesa rapid evaporation of moisture from the substance producinga low temperature of the substance which may fall below the freezingpoint of the substance. 'Ihe rate of evaporation may be increased byoperating the agitator I4 and the temperature of the substance may becontrolled by the application of heat tothe kettle.

Refrigerant is circulated through pipes and 32 to maintain the innersurface of condenser section 26 at a desired low temperature which isless than C. and lbelow the vapor pressure of the evaporating moisture,preferably from 40 C. to 70 C. The evaporating vapors flow from outletport I8 of the kettle through conduit portions 22, 23 and 24 and betweenthe spokes of spider d2 into the condenser section 26 where theycondense and freeze to solid form on the inner surface of the condenser.Rotor 36 is continuously operated by motor 46 causing blades -58 toscrape the condenser surface to remove the solidified vapors or ice, theiceparticles falling through the section 26, spider 44 and port 60 intocontainer 62.

The operation as so far described is essentially the same as that of theapparatus of application Ser. No. 483,274 previously referred to.However, we have found that the vertical arrangement of the condensercylinder herein is an improvement over the inclined cylinder shown inthe aforesaid application. In the latter, the rotary movement of thehelical blades is relied on to remove the bulk of the ice particlesscraped from the condenser surface from thelower end of the condenser.This increases the load on the scraper and reduces its eiiicincy. Thepresent arrangement permits the ice particles to drop directly out ofthe condenser into the ice .receptacle, thus reducing the load on thescraper. Also, we have found that the straight scraper blades and theirsupporting rotor herein dej scribed form a stronger, -more efllcientscraper unit than the helical blade assembly disclosed in the saidapplication.

The walls of the ice receptacle are kept at a carefully controlled lowtemperature by circulation of a refrigerantv within the jacket 84. To

avoid preferential condensation of vapors on the mation of vapors fromthe ice passing tothe receptacle from the preceding condenser and toattain this purpose we have found that the temperature of the receptacleshould be no more receptacle. The auxiliary condenser is kept atsubstantially the same low temperature as the main condenser,- but hereagain care is taken that the temperature does not dropv below that ofthe main condenser .to prevent the vapors bypassing the main condenserand freezing preferentially in the auxiliary condenser.

While the system is `in operation, door 'wand v cover l2 are locked inposition and the space between Vthem is evacuated to effectivelyinsulate the outer surface of door lll. When it is desired to remove theice from the receptacle, pipe 82 is closed and cover 12 and door 10 areremoved. The accumulated ice may then be withdrawn from the systemthrough port B8. To prevent loss of vacuum in the entire system on suchoccasions, suitable valves may be provided in the inlet and outlet portsto the receptacle. so that it may be temporarily closed of! from therest of the system.

Having described a preferred form of the process and apparatus of ourinvention, what we desire to claim and secure by Letters Patent is:

1. In a process for the rapid desiccation of substances such as foodproducts, chemical product-s, plasma and the like under sub-atmosphericof said system in proximity to said removed solidified vapors within thesystem to controlled `l refrigeration suchas to maintain their innersurface at a temperature within the range from la than 10 C. higher thanthat of the preceding condenser. Our preferred practice is therefore tokeep the ice receptacle at a temperature equal to or not more than 10 C.above the temperature of the condenser 26. Any vapors which do exist inthe receptacl are prevented frornescaping with the free-air to themechanical pump by the auxiliary condenser 92, in the outlet to thepump, which freezes and traps them. Scraper unit IM is continuallyoperated to remove the frozen vapors from the surface of the auxiliarycondenser from which. as solid particles, they fall back4 into thetemperature 10 C. above to a temperature equal to the temperature of thecondenser surface.l

2. In a process for the rapid desiccation of substances such as foodproducts, chemical products, plasma and the like under sub-atmosphericfree air pressure in an hermetically sealed duct system connected to afree air pressure reducing means,

the steps which comprise condensing and freezing to solid formthe vaporemanating. from the substance upon a pair of cold surfaces spacedlongitudinally of the system between the substance being desiccatedtherein and said pressure reducing means, said surfaces being maintainedat a temperature below 0 C.' and substantially below thetemperature ofthe substance and thev one of said surfaces nearest said substance beingat least as cold as the other one of said surfaces, removing solidiedvapors in solid form from each of said surfaces, and subjecting thewalls of said system` in proximity to said removed solidified vaporswithin the system to controlled refrigeration `such as to maintain theirinner'surface at a temperature within the range from a temperature 10 C.above to a temperature equal to the temperature of the one ofI saidcondenser surfaces nearest the substance being. desiccated.

3. In a process for the rapid desiccation of substances such as foodproducts, chemical products, plasma and the like under sub-atmosphericfree air pressure in an hermetically sealed duct system connected to afree air -pressure reducing means, the steps which comprise condensingand freezing to solid form the vapor emanating from the substance upona-cold surface within said sys- 'tem maintained at a temperature below0C. and

substantially below the temperature ofthe subaccadenstance, removingsolidied vapors in solid form from said surface while maintaining thesuriace at condensing temperature, passing the ice so removed t areceptacle in said system, and maintaining said solidiiied vapors insaid receptacle at. a controlled temperature low enough to preventsubstantial sublimation thereof at the. pres sure maintained in thesystem by circulating-a fluid refrigerant in indirect contact, through aheat conducting wall, with the interior of said receptacle.

4. In a process for the rapid desiccation of substances such as foodproducts, chemical products, plasma and the like under sub-atmosphericfree air pressure in an hermetically sealed duct system connected to afree air pressure reducing means, the steps which comprise condensingand freezing to solid form the vapor emanating from the substance upon acold surface within said system maintained at a temperature belowiio C.and substantially below the temperature of the substance, removingsolided vapors in solid form from said surface while maintainingsub-atmospheric pressure in the system and maintaining the surface atvapor-condensing temperature,

passing the solidified vapors so removed to a. re-

vceptacle in said system, and maintaining said solidliled vapors in saidreceptacle at a controlled `temperature low enough to preventsubstantial sublimation thereof at the pressure maintained in the systemby circulating a fluid refrigerant in indirect contact, through a heatconducting wall,

with the interior of said receptacle. l i 5. In apparatus for thedesiccation `of substances under sustained `low pressures which includesa free air pressure reducing means and an.

l. hermetically sealable duct system connected to d from a saidsubstance in said container, means for refrigerating said surface to atemperature low enough tov cause vapors to condense and freeze' tosubstantially solid form thereon at the pressure maintained inthesystem, a receptacle in said system adjacent said condenser, meansfor removing solidified vapors in solid form from said condenser surfaceto said receptacle, means for operating said removing means while saidsystem l is hermetically sealed and a low free air pressure ismaintained therein by said pressure reducing means, and means forcirculating a uid refrigerant in indirect contact, through aheatconducting wall, with 'the interior of said receptacle to maintainthe solidified vapor therein at a temperature low enough to preventsubstanu tial sublimation thereof at the low pressure maintained in thesystem.

6. Apparatus as claimed in claim 5 wherein said condenser comprises asubstantially cylindrical and vertically disposed conduit connected toand disposed above the receptacle, the inner surl face of said conduitforming the condensing surface of said condenser, and the solidifiedvapor removing means comprises a rotary support within said conduitcarrying a. substantially straight scraper blade extendinglongitudinally of the con duit and having its outer edge in closeproximity closure means from said outer closure means.

NORMAN V.' HAYES. WILLIAM B. HUMES.

